This invention relates to a plasma reactor apparatus and more specifically to a plasma reactor apparatus which provides improved uniformity of etching and improved overetch control.
The use of plasma processing is gaining rapid acceptance as a replacement for older conventional processing. This is especially true in the semiconductor industry but is becoming equally true in other manufacturing fields as well. Plasma etching is being used in place of wet chemical etching and plasma deposition is being used in place of high temperature thermal deposition. Plasma processing offers advantages in cost, environmental impact, and repeatability.
As the use of plasma processing increases, there is a growing demand for uniformity and repeatability of the plasma process. Uniformity is essential across a single workpiece and from workpiece to workpiece. In the semiconductor industry, for example, the etching of fine lines or spaces across a semiconductor wafer must be uniform over the total area of that wafer in order to insure adequate yield of all devices on the wafer. This requirement for uniformity across the wafer becomes more stringent as the semiconductor industry matures and the designed values of lines and spaces are reduced. Repeatability from wafer to wafer is important, again to allow high yields. Processing lots do not always stay constant throughout the device process; that is, an insulator layer, for example, may be deposited on small batches of wafers which are later combined to form one large lot. As this lot then proceeds through subsequent processing steps, it is imperative that the subsequent steps be uniform and predictable.
Existing plasma reactors can be roughly divided into two types: barrel type reactors and parallel plate type reactors. In the barrel type reactor, workpieces are loaded into a cylindrical reaction chamber and a reactant plasma is introduced into that chamber. The plasma is created by a field from an electrode which surrounds the workpiece. Gas flow is approximately axial along this type of reactor and may be improved by an injection manifold which injects gases more or less uniformly along the axis. This type of reactor, however, suffers from two types of non-uniformities. One of these non-uniformities results from the external electrode which cannot provide a uniform field with respect to workpieces within the chamber. The other results from the gas flow kinetics. The plasma tends to flow through the reactor in a non-uniform manner and suffers, additionally, from plasma depletion effects. Parallel plate reactors provide a degree of improvement in uniformity over the barrel reactor by providing a more uniform and better defined field at the workpiece. The parallel plate reactor, however, still suffers from non-uniformities resulting from a non-uniform and usually radial reactant gas flow. These non-uniformities have been markedly improved in the reactor apparatus disclosed in co-pending application Ser. No. 847,349 filed Oct. 31, 1977, and assigned to the assignee of the present invention. That apparatus provides means for uniform injection of reactants into the reaction space between closely spaced parallel plates. Even with this configuration, however, non-uniformities exist because of the non-uniform removal of reaction products from the edge extremities of the reaction volume.
All of the aforementioned plasma reactors have an additional fault. The total reactor, including a considerable amount of unused volume, is filled with the reactive plasma although the workpieces occupy only a limited amount of that volume. In the cylindrical reactor, the total cylindrical chamber is filled with plasma. In the parallel plate reactor, even the volume outside the plates is filled with plasma. Filling this "dead space" with plasma is uneconomical because of wasted reactants and especially because of the extra power required to maintain this unused plasma. The power requirement is important because a given power supply must be large enough to generate both the used and unused plasma. The energy of the used plasma is unnecessarily limited because the power supply must also generate the unused or wasted plasma. More importantly, as the pressure in the reaction chamber changes the plasma volume changes, expanding unpredictably into the dead space. This presents problems with reproducibility from run to run.
Accordingly, it is an object of this invention to provide a plasma reactor apparatus which allows an enhanced uniformity across the workpiece and from workpiece to workpiece.
It is a further object of this invention to provide a reactor apparatus having a decreased "dead space".
It is a still further object of this invention to provide a plasma reactor apparatus which provides improved dimensional control of the plasma process.
It is another object of this invention to provide a plasma reactor apparatus which minimizes the standard deviation of undercutting dimensions with overetch time.
It is still another object of this invention to provide a plasma reaction process having improved dimensional control.